1. Field of the Invention
The present invention relates to a process for the purification by an organic solvent mixture of an aqueous phosphoric acid which was obtained by the wet process. The process is carried out by the counter-current extraction principle in a plurality of stages.
2. Description of the Related Art
Wet-process phosphoric acid, in the present invention, is taken to mean an aqueous phosphoric acid which has been prepared by decomposition of crude phosphate using acid, preferably sulphuric acid, and which has a P.sub.2 O.sub.5 concentration between 40 and 62% by weight. An acid of this type contains a number of inorganic impurities which interfere with the further use of this acid and must be removed.
To produce pure phosphoric acid by extraction with organic solvents which are water-immiscible or only partially miscible with water, a large number of processes have already been proposed. In the processes which have been disclosed, solvents such as alcohols, ketones, ethers and esters of phosphoric acid are used.
However, there are only a few proven processes in practical use. All processes have the disadvantage that the solvents used are not able to take up the water present in the wet-process phosphoric acid. This means that the degree of extraction, i.e. the yield of phosphoric acid that can be achieved, is determined by the decreased water uptake capacity of the solvent. For this reason, the circulated partially water-miscible organic solvent, in particular the alcohols, must be completely or partially dehydrated. Processes of this type are described, for example, in DE-C 23 21 751, DE-C 23 34 019, DE-C 21 27 141 and DE-C 26 57 189. The degree of extraction is known to be dependent on the P.sub.2 O.sub.5 concentration of the crude acid, on the number of extraction stages, on the solvent to acid ratio and on the amount of recycled wash water.
In EP-A1 0 111 802, the partition coefficient when anhydrous alcohol is used is comparatively greater than when water-saturated alcohols are used. The P.sub.2 O.sub.5 yield is here only 50 to 60% by weight. Nevertheless, this makes little difference to the fact that to achieve maximum yields, a high number of extraction stages are required and, at the same time, a high solvent to acid ratio of 8:1 to 20:1 is required.
Wet-process phosphoric acid may also be extracted by the so-called splitting process. This process is based on the fact that only a proportion of the P.sub.2 O.sub.5, approximately 40-60% by weight, is extracted from the crude acid.
A typical procedure is described in U.S. Pat. No. 3,318,661. According to this, the crude acid containing approximately 48-62% by weight of P.sub.2 0.sub.5 is extracted with isopropyl ether in a weight ratio of 1:0.5 to 1.5, about 40% of the P.sub.2.sub.5 passing into the organic phase. The purified phosphoric acid is produced from the isopropyl ether extract by adding sufficient water for an aqueous phosphoric acid solution having a P.sub.2 0.sub.5 content of 48% by weight to be separated out from the organic phase. It is a disadvantage in this process that the isopropyl ether extract cannot be washed and therefore only a low-quality phosphoric acid can be prepared. It is not possible to set a higher acid to solvent ratio to achieve a higher selectivity, since a third phase is then formed.
A further disadvantage of this process is that the crude acid must be concentrated from approximately 48% by weight of P.sub.2.sub.5 at least to 57% by weight of P.sub.2 0.sub.5, in order to achieve a P.sub.2.sub.5 yield of 60%.
Improvement in selectivity has been described in U.S. Pat. No. 3,903,247, according to which the crude acid is extracted with a solvent mixture consisting of 75-95% of a water-immiscible C2-C5 ether and 5-25% of a lower C3-C8 alcohol. A particular disadvantage of this process is the poor yield and also, especially, the fact that an excessive amount of acid remains in the raffinate. The quality of the acid that is prepared must be classified principally only as "technical grade", i.e. it is not possible to prepare a higher quality without great expenditure on equipment.
Further extraction processes are described in DE-A 2,320,877 and U.S. Pat. No. 3,556,739, in which dialkyl ketones are used as solvent in a weight ratio to the acid of 0.5 to 2.0:1. An advantage of this process is the fact that the ketone extract, in contrast to the ether extract, can be purified by washing with water or phosphoric acid. The quality of the purified phosphoric acid is moderate, for which reason a further more complex purification step by concentration and crystallization of the phosphoric acid must also be carried out.
The object was therefore to eliminate the disadvantages occurring in the known extraction processes, in particular to prepare a phosphoric acid in high purity and at high yield. Such high-purity phosphoric acids are required in the food industry, for example.